It is desirable to remove particles from a fluid for improving cleanliness or safety of an environment, for removing undesirable particles that otherwise may interfere with efficiency or operational life of a device, or for other purposes. It is also important in many applications to remove particles in an efficient and economical way, and to remove particles having a relatively small size.
There are many existing methods for removing particles from a fluid (for example, air). Contact-based methods may be used, such as fiber and other media filters and water or oil scrubbers. Another particle removal method employs electrostatic precipitators. However, such methods often require relatively large quantities of airflow through filtration media so that particles may be captured and removed from the air. Fiber and electrostatic filters, for example, require a medium for making physical contact with the particles to remove them from an air stream. Particles quickly accumulate onto the filtration media, and thus the media is cleaned or replaced frequently, resulting in high maintenance costs. For many dusty environments, such an air cleaning process requires frequent maintenance, including replacement of filters.
Other, non-contact air cleaning devices remove particles from a fluid aerodynamically, rather than by passing the fluid through a filtration media. Certain non-contact devices use centrifugal force to separate particles from a main air stream. These so-called cyclones create spiral airflow at a very high speed to separate particles from the air. The tornado-like particle separation process involves no contact media, and thus does not require frequent cleaning or replacement of a filter media. Additional non-contact devices include louver and baffle types, and settling chambers.
There are two principal types of cyclone air cleaning devices: return flow and uniflow. A return flow cyclone allows air to return, while a uniflow cyclone does not. Due to differences in airflow between these two types of devices, the particle separation processes are quite different.
A large amount of research has been conducted for return flow aerodynamic air cleaning devices, for example, for air sampling purposes, while a relatively smaller amount of research has been conducted for uniflow cyclones. However, though return flow cyclones allow a small particle cutsize (the size of a particle for which collection efficiency is at least 50%), they are energy intensive and have low dust separation efficiency. Furthermore, airflow velocities are very high in conventional cyclones. Accordingly, high turbulence and reentrainment of particles occur, resulting in low particle separation efficiency, especially for small particles.
Traditional cyclone particle separation devices have exhibited great difficulty in separating particles smaller than 10 μm from air. Separation of only larger particle sizes, however, is not particularly useful for conventional air cleaning purposes.